Process for polymerizing ethylene



arch 14, 1937 J. M. KELLEY ETAL 3,309,35

PROCESS FOR POLYMERIZING ETHYLENE Filed July 31A, 196s United States Patent 3,309,350 PROCESS FOR PULYMEREZING ETHYLENE Joseph M. Kelley, Westfield, John J. Miskel, Jr., Ramsey,

and Paul J. Marinaccio, Dumont, NJ., assignors to Rexail Drug and -Chemical Company, Los Angeles,

Calif., a corporation of Delaware Filed July 31, 1963, Ser. No. 299,028 7 Claims. (Cl. 26094.9)

This invention relates to a process for polymerizing ethylene to high molecular weight linear polymers of controlled melt viscosity and high densities and it relates in particular to a high catalyst efficiency process wherein extremely high quantities of poly-mer are prepared per pound of catalyst used so high in fact are the quantities of polymer produced per pound of catalyst used, that where specific conditions of this process are employed, the polymer can be recovered directly from a slurry and the costly step of deashing (catalyst residue removal) eliminated entirely.

One of the disadvantages of prior art polyethylene manufacturing processes with titanium tetrachloride and aluminum trialkyl at pressures ordinarily below 500 p.s.i.g., has been the fact that the ultimate polymer recovered from a slurry or solution is contaminated with catalytic residues such as titanium, aluminum and chlorine. Titanium is particularly bad as a polymer contaminant since amounts anywhere above 50 to 60 parts per million based on the polymer (and in some cases above 20 parts per million), if not deactivated in some manner, usually have to be removed therefrom, otherwise upon processing of the polyethylene, such as in injection molding, the polymer will discolor or streak and as a consequence, will be downgraded.

Although efficient deashing schemes have been developed in the prior art, such deashing schemes usually require a large plant investment, for example, in the order of about 20% and this adds considerably to the cost of manufacturing the polymer.

Athough it has been reported in the art that some catalysts other than those with which this invention is concerned give high catalyst efficiencies in vapor phase reaction, thereby not requiring the removal of ash residues, usually the polymers obtained are of very high molecular weight and extra steps are required to bring the molecular weight of the polymer to a suitable range for commercial plastic processing applications. Such steps may include thermally degrading the polymer or blending of the polymer with lower molecular weight material, in any event, requiring some special aftertreatment, thereby adding to the total cost of manufacture. A process wherein an ethylene polymer can be prepared without removal of catalyst residues and without special aftertreatment to reduce molecular weight has not heretofore been reported in the industry.

In prior art commercial processes for preparing linear polyethylene employing catalysts produced by aluminum alkyl reduction of titanium tetrachloride (the usual brown catalysts) there are consistently obtained linear polyethylenes of densities ranging from about 0.945 to 0.955. Thus, in M. Sittigs book, Polyolen Resin Processes, p. 5, 1961, Gulf Publishing Company, Houston, Texas, there is illustrated in graph form densities of various commercial linear polyethylene resins and the processes used in manufacturing them. It is illustrated, for example, that linear polyethylenes prepared by the so-called Ziegler Process may range in density from about 0.945

ice

to about 0.955. 4Polyethylenes prepared by the Phillips Process on the other hand, that is, with a chromium oxide containing catalyst composite, range in densities from above, for example, 0.955 to 0.965 or higher. It is evident that it would be both desirable and highly advantageous to consistently produce linear polyethylenes of densities higher than 0.955 by the use of titanium containing catalysts. The process of this invention now provides this desirable feature, so that linear polyethylenes having densities of 0.96 and higher can be produced while employing titanium containing catalysts as will be illustrated hereinbelow.

An object of this invention is to provide a process for preparing an ethylene polymer wherein such high catalyst efficiencies are obtained that the polymer need not be deashed.

A further object of this invention is the provision of a process for preparing an ethylene polymer wherein molecular weight control can be achieved during the polymerization of ethylene.

A still further object of this invention is to provide a process for preparing linear polyethylenes of densities of above 0.955 by the use of a titanium containing catalyst.

A still further object of this invention is the provision of a high catalyst eihciency process for preparing polyethylene possessing certain economic advantages over prior art techniques and resulting in polymers containing substantially no catalyst residues.

Another object of this invention is the preparation of copolymers of ethylene with alpha-olefin monomers in a high catalyst etiiciency process.

In accordance with the foregoing, it has been discovered that ethylene can be polymerized or copolymerized in a high catalyst efficiency process by reacting ethylene alone or in the presence of minor amounts of an alpha-olefin monomer in a polymerization reactor at pressures of at least 750 p.s.i.g. and temperatures of from 50 to C. in the presence of an inert hydrocarbon diluent boiling below 10 C. while employing as the catalyst for the polymerization reaction titanium trichloride and as an activator therefor, an aluminum compound containing at least one metal to carbon bond, the ratio of aluminum to titanium employed being at least 2:1, but preferably about 5:1, and more preferably 25:1 to as high as :1. By the process of this invention, specilically in the preferred embodiment of this invention, there is obtained ethylene polymers or copolymers having a minimum residual titanium content in the order of 10 to 15, and generally below 60, parts per million, thereby requiring no deashing, and in the case of homopolymers, linear polyethylenes of densities higher than 0.955 and up to 0.97.

The process of this invention also provides in its preferred embodiment, a technique for carrying out the high catalyst efficiency polymerization and this comprises the steps of introducing ethylene and hydrogen to a polymerization reactor containing therein as an inert hydrocarbon diluent, a hydrocarbon having a boiling point of from about 45 to 0 C. employing pressures in said reactor of at least 750 p.s.i.g. and up to 10,000 p.s.i.g. but preferably 1000 to 4500 p.s.i.g. at temperatures of from about 50 to 100 C. In the preferred embodiment hereof, the ethylene, after a conversion of from 50 to 95 percent in said reactor, which conversion is carried out in the presence of titanium trichloride activated with an aluminum trialkyl, the aluminum to titanium mole ratio being about 25: 1, is withdrawn from the reactor to a lower pressure zone in the form of a slurry of ethylene polymer, unreacted ethylene and hydrocarbon diluent and continuously ashed from said slurry. The flashed ethylene and inert hydrocarbon diluent is recycled directiy to the polymerization reactor, or where preferred, a simple clean-up treatment is provided for this recycle stream. The ethylene polymer substantially completely dry from the low pressure zone, can be led either directly to an extruder or it can be steam stripped and then extruded and pelletized. In the preferred process as set forth, from to 80 mole percent hydrogen based on the ethylene can also be introduced into the reactor to control molecular weight of the polymer.

It was unexpected to find that ethylene could be polymerized at such high catalyst efficiencies with a titanium trichloride catalyst component, specifically where hydro een is included as a molecular weight 4control agent. It was further unexpected to find that homopolymers of ethylene were consistently produced with densities of 0.96 and greater. The process of this invention is not only applicable to homo-polymerization of ethylene in hydrocarbon diluents such as ethane, propane, butane, isobutane or neopentane, but preferably propane or butane, but also to copolymerization of ethylene with other alpha-monomers by the employment of pressures above 750 p.s.i.g. It was found that if pressures below 750 p.s.i.-g. are employed, while good polymerization rates are obtained, the residual ash content of the polymer remained high. No explanation can be offered for the increased catalyst eiciency at pressures higher than 750 psig., although, as will be illustrated in the examples, at pressures at low as 800 p.s.i.g., catalyst efficiencies of consistently above 60,000 and usually up to and higher than 100,000 pounds of polymer per pound of catalyst based Aon titanium have been realized. With these etliciencies, and specifically those approaching 100,000 pounds of polymer per pound of titanium based on the metal, the residual polymer contains on the order of 10 parts per million titanium, thereby requirnig no deashing. It should be pointed out here that for some applications titanium content in parts per million of above 15, for example, up to 60 `can be tolerated. Thus, even in some of the best known deashing schemes, it is not unusual to detect titanium contents of this order. The process of this invention lobviously possesses superior advantages over those which require such after treatment to remove titanium residues.

It has been known to polymerize ethylene or propylene in normally liquid diluents such as hexane or heptane and to control the molecular weight or viscosity of the polymer by the use of hydrogen. It is also known that the use of hydrogen deactivates to a minor, but important, extent the catalyst used in the polymerization of either ethylene or propylene. This is particularly true where titanium trichloride catalysts are employed in propylene polymer processes. For example, in British Patent 908,101, it was disclosed that hydrogen can be satisfactorily used for control of molecular weight of polypropylene and that certain low melt viscosities can be obtained. It was also disclosed in this patent, however, that in order to obtain polypropylenes of lower melt viscosity, more hydrogen must be used than when the higher melt viscosity polypropylenes are prepared, but the rate of the polymerization reaction and the yield of polypropylene produced were reduced when using these higher quantities of hydrogen. This observation has been found to :be true especially where hydrogen was used as a molecular weight control agent in ethylene polymerizations at pressures below 750 p.s.i.g.

While the deactivating effect of hydrogen continues even with pressures above 750 p.s.i.-g., it was found that upon elevating the polymerization pressure of the reaction system to preferably from 1000 to 3500 p.s.i.g., that 10 to 50 percent hydrogen based on the ethylene feed did not reduce the catalyst efficiency suliciently to require deashing of the polymer.

The process of this invention is applicable to the polymerization of ethylene with minor amounts of comonomers such as propylene, butene-l, pentene-l, hexane-1, octene-l, etc., as well as branched alpha-monomers such as 4-methyl-pentene-l, 4,4dimethyl-pentenel, S-methylbutene-l, 3,3-dimethyl-butene-1 and the like. The amounts of alpha-olefin monomer fed with ethylene can range from 1 to 20 mole percent based on the ethylene feed.

One of the unexpected and added advantages of employing, particularly butene-l as a copolymerizable monomer in the process of this invention was the finding that when butene1 is used to contact the catalyst prior to the polymerization that a pronounced effect on the polymerization rate is obtained. For example, by treating the mixed catalyst with butene-l for a certain period of time and temperature, the rate of polymerization of ethylene is substantially doubled. This treatment can be carried out with the mixed catalyst, for example, by adding an amount of from about 0.2 to 3 grams of butene-l per miligram of TiCl3 catalyst. The treatment can be carried out in the inert diluent merely by adding the butene-l to the mixture and heating at 25 to 75 C. for .about 1 to 30 minutes, but preferably at a temperature of from 50 to 65 C.

In accordance with the foregoing, a broad pressure range for carrying out the polymerization reactions of this invention is from at least 750 p.s.i.g. and as high as 10,000 p.s.i.g., but preferably from 1000 to 5000 p.s.i.g. and more preferably from 2000 to 4500 p.s.i.g.

A preferred catalyst for carrying out the process of this invention is titanium trichloride, specifically the violet form and more specifically a titanium trichloride cocrystallized with a Group Il metal halide, for example, magnesium, zinc or aluminum trichloride, with aluminum trichloride being preferred. In the preferred form for this catalyst compound, nTiCla-AlCl3, the letter n represents a digit that can vary from 1 to 5. In a specific embodiment of this invention the cocrystallized titanium trichloridealuminum chloride is subjected to attrition such as ball milling in .an inert or dry atmosphere, activated with aluminum triethyl and employed as the catalyst in the process herein to obtain very high polymerization rates. Although aluminum triethyl has been specified as a particular catalyst component preferred, aluminum trialkyls or dialkyl aluminum mono halides can likewise be employed. The alkyl groups can contain from 1 to 10 carbon atoms, lmore specifically from 2 to 8 carbon atoms .and the preferred halide is chlorine.

The amount of total catalyst preferred per part by weight of ethylene introduced into a reaction ranges from 0.0001 to O. 001.

It is preferred, according to this invention to employ an aluminum to titanium ratio of at least 2: 1, more preferably 2511 to as high as :1 for example. In processes of this nature, it is not required that both catalyst components be added simultaneously in the ratio stated, since it has been found particularly advantageous to treat the inert diluent with aluminum trialkyl to deactivate any catalytic poisons therein and thereafter to add the requisite amount of TiCl3 and/ or aluminum triethyl to bring the catalyst ratios to those preferred.

Usually the preferred inert diluents herein, such as propane or butane, contain small amounts of poisons such as Water and these poisons must be removed or deactivated prior to use of these diluents as polymerization media, otherwise, because of the small quantity of catalyst required in the polymerization, such poisons tend to destroy at least .part of the catalyst activity. It is, therefore, preferred to treat the butane diluent, for example, with 0.001 to 0.003 part aluminum trialkyl per part by weight of butane diluent. Since suicient aluminum alkyl remains in the diluent, it is therefore only required to add sufficient titanium trichloride alone or in combination with aluminum trialk'yl to bring the combined catalyst to the desired Al/Ti ratio. The preferred dil-uents as indicated herein are propane and butane.

o sicant adsorbents, the recycled stream is taken via line 27 to storage vessel 28 or directly from storage vessel 28 to line 29 to reactor 10. Fresh diluent can be added through line 30.

The most eiicacious temperatures to be used in this From cyclone 20, which can be suitably arranged with process are temperatures of from 50 to 100 C. with best a heat loop (not shown) to maintain desired temperatures, results being obtained at temperatures of 65 to 80 C. a substantially dry ethylene polymer is recovered land As indicated hereinabove, hydrogen can be used as the this can be taken directly to a drier and an extruder or molecular weight control agent in amounts of from it can be steam stripped by removing the polymer from to 80% based on the ethylene feed, -but preferably where 10 cyclone 20 through line 31 and introducing it to vessel an alpha-comonomer is employed, a combination 5 to 32. In vessel 32, steam is introduced through line 33 25 percent hydrogen and 1 to 20 percent comonomer. to treat the polymer and is then removed through line To obtain high catalyst eiciencies in the process of this 341-. The steam stripped polymer can be removed from invention, it is required that the residence time in either vessel 32 through line 35 and conveyed to a drier system a batch or a continuous process for ethylene polymerizaindicated generally at 36. From 36, the polymer powder tion average from about 1 to 5 hours in a reactor. can `be taken to extruders or other finishing steps not Where such residence times are employed, Ipolyethylene shown. The following is an example of the operation is produced containing less than l5 parts per million of of this process. titanium and sutiiciently low aluminum and chlorine con- Example 1 tent so that deashing is not required.

The process of this invention in its preferred embodi- In operation, the ProeeSS of this invention iS Carried ment is depicted in the attached drawing which forms out in UnitS `21S Shown in the drawing by oontinuousiy in' part of this invention and illustrates a process flow diatrodlleing 45 ibS-/hour of etiiyiene (to form 125 ibS- of gram for carrying out the polymerization reaction. Poiyetiiyiene in the reactor) to reactor it) Containing In the drawing, reactor 10 is a pressure resistant vessel 25 250 ibs. Of butano iiqllid, adding 15 to 30 rnoie Percent capable of withstanding pressures up to 10,000 p.s.i.g. or hydrogen and further adding to Said rniXture Catalyst in higher Ethylene is introduced t() the reactor through an IIlOLlIli. Of 0.005 lb. fOr the titanium tfChlOIlde and line 11, aluminum trialkyl through line 12, titanium tria Suincient quantity of aluminum triaikyl t0 the -butane chloride through line 13 and recycled inert diluent diluent to obtain an Al/Ti ratio of 50 to 1. The polymthrough line 29. Due to the nature or the inert diluent erization is carried out continuously until a conversion used, the exothermic reaction causes the boiling of the of ethyiene of from 50 to 95 Percent requiring a residence diluent and through evaporative cooling, a certain amount time of about 3 hours and 3 85 ibs of Siiirry nre With* of heat can be removed from the reactor. Diluent vapors, fawn through hhe 1h Chlhphsed Of 125 lhs' of Poiyiher as well as unreacted ethylene (and alpha-comonomers) 250 ibs' of hhiahe ahh i0 ibs; of eiiiyifa'he' ,A Siighi and/or hydrogen are removed through line 14, condensed aiiioiiiii Oi iiyiiiogeii is aiso iiiciuded iii .this Siuriiy' Through operation of the let-down valve 19, mtermlttent- 1n condenser 15 and recycled through line 16 and pump 1 t. I 1 tb 1 .n 10W d to mos hec 17 to reactor 10. Compression eeuipment (not shown) y or con muOLSy. ie s urry id fere p r or below pressure m cyclone-bag nlters 20 and 21. One ciiii he pioviiieii for iecyciiiig Oi iiyiiiOi-ieii iiiiOiiiiicaiiO-iis hundred twenty-ve lbs. of polyethylene are recovered of this arrahgeihhhi chh he ihade'ii hesiifd' 40 from the cyclone through line 31 containing about 2.0

Ahfi'r ahi/0 Or three hhhr iesidehc time 1h the Phil/m' percent volatiles. The volatiles flashed from the slurry erization reactor and a preferred conversion of from l0 which amount to 2475 lbs. of butane and 10 lbs. of unto 40 Weight Percent SoiiCiS, an etiiYiene-inert diiilent reacted ethylene are recycled tothe reactor as illustrated slurry is let down from line 1S and valve 19 to atmosin the drawing, pheric pressures or slightly above in cyclone-bag filters The conditions in the above example are as follows for 2) and 21. the various units in the process:

Reactor Pressure, p.s.i.g., 3,000- Temperature 80 C.

Cyclone-bag Filter.

Pressure, p.s.i.g., Temperature 40 C.

Amount of Polymer.. 125 lbs Catalyst Efficiency-.- 100,000 lbs/lb. based on Ti metal. Polymer Density 1. 0.S6+g./cc Ash Residues Ti 10 ppm. Melt Index (range) 2 0.2 to 1.0

l Density is determined according to ASTM D-1505-T. 2 Melt Index in grams/10 minutes is determined according to ASTM D-1238-57T, and can be controlled for example to produce higher values than above by using more hydrogen.

The vaporize-d ethylene, inert diluent and small amounts of hydrogen are taken via line 22, heater 23 and accumulator 24. Suitable compression equipment (not shown) can be associated with this arrangement to condense butane if needed. A certain amount of the reactants are vented from accumulator 24, for example, from 1.0 to 5.0 percent in order to reduce the inerts or impurity level, if any, in the reaction system. The vented gaseous materials can be taken to purification units (not shown) and to recycle. The recycle stream is then taken through line 25 to either of two dessicant adsorbents 26 in series for removal of any impurities picked up upstream of this unit if desired. Suitable desiccant adsorbent materials The following denitions are to be noted:

(a) Rate of polymerization w./hr./w.=weight of polymer per hour per weight of catalyst (b) Catalyst eiiiciency=lbsof polymer produced per pound of TiCl3 catalyst yadded (or per pound of Ti metal added).

The above values are calculated by two methods. In the first method, rate or catalyst eiciency is calculated based on the actual lbs. of catalyst added and the lbs. of polymer produced. Inl the second method, which is a check on the catalyst eiciency calculated by the first method, the undeashed polymer is analyzed for Ti metal and using this value, the catalyst eiciency is back calare molecular sieves, silica, alumina, etc. From the desculated.

In Example 2, Table I and the remaining examples below, a series of runs were made using a one liter autoclave containing 500 cc.s of liquid butane at the conditions indicated in the table. The ethylene was pres- Example 4 Two runs were carried out at 800 p.s.i.g. as reported in Table III below, one using hydrogen and the other omitting it.

sured into the reactor and catalyst components added by 5 s rin e butane was retreated with aluminum trieth l y p y TABLE rrr as 1n icated heretofore). The remainder of the aluminum triethyl was added along with TiCl3 to lbring the mixture Run 6 7 to the desired Al/Ti ratio.

lo Tioi., grams 0.0i 0.0i Example 2 iI/ifrinri Rang-.. 50 s0 uent utaiic 00.5. 500 500 TABLE I Time min.) 50 15s Temperature, C 80 80 Pressure, p.s.i.g 800 800 Rllll Dumber 1 2 3 4 1 H2 in Feed, mole persen 0 9.2 Yieid, grams 271.4 2.54 l Rate of Polymerization, lbs./hr./l .0 a ca a ys 380 354 Al/T1Rat1o 140 7 70 35 Catalyst Eicieney, loJlb. on TlClg 27, 140 25, 400 Piessuie, psi.;y 1,000 i, 000 1,000 1.000 Catalyst nfneeney, ib./ib. on 'ri metaL. 108, 000 102, 000 Hg, percent on Ethylene Fee 0 11i. 1 37. 2 Melt Index, gjm min. 0.064 Yield, grams 68 79 72 53 Density, gjcc 0.0640 Melt Index, g./10 min- 0 0. 16 1.7 Intrinsic Viscosity, deeilitcrs/gmin 3.52 Density, glee 0.9605 0.0035 Rate oi Polymerization, w.,'lir./w. y C0i1'ii1f T 27, 200 05 000 14,400 5.300

ala. ys lieiency w. W. on i Civieltaiaiflged). f 114,000 05, 500 00,300 22,000 Run 7 above shows that at catalyst eh'iciencies of as ata yst iciency w. w. (on Ti Measuredm www, u 101,000 61500 ,S1200 high as 102,000 lbs/lb. on Ti metal. that densities above Fraai Ti coment 0f Poiymer (by 0.96 are obtained by the process herein.

analysis), p.p.m 10 16 55 20 Example 5 All runs 1 hour except number 2 which was 10 minutes.

Table lV below shows copolymer runs employing ap- The above data show that at pressures of 1000 p.s.i.g., paratus as in the above examples in which high catalyst high molecular weight linear polyethylene can be made eiliciencies were likewise demonstrated.

TABLE IV Run s 0 10 1i 1 Grams T1013 .0i .0020 .005 .005 Time- 1.7 1. 1.0 1.0 Hq, rnole percent in feed 7. 4 Ai/Ti ratio 140 7o 70 Pressure, 10.5.1.0. 1,000 1, 000 1, 000 1, 000 Butene-l, 00.5 25 25 25 25 Yield, grams." 200 64 7S 1 Eciency ltr/lh., on Ti metal. 66, C07 00, 909 G5, 000 -100, 000 Ti Analysis, p.p.in 15 10 Melt Index, g./10 min. 04

l Pretreatment step as follows-Butene-l added to mixed catalyst in hutane at C. and heated for 9 minutes at 65 C.

at catalyst efficiencies of 100,000 w./w. on Ti metal, giving a residual Ti content of about 10 ppm. in the polymer. The use of high amounts of hydrogen as in Run 4 tend to decrease catalyst efficiency, but this could be counteracted where pressures higher than 1000 psig.

are employed, for example, 2500 to 3000 p.s.i.g., as shown in Example 1.

Example 3 In a further run using a small scale reactor (one liter autoclave, as in Example 2) and hydrogen to control molecular weight, a high catalyst efliciency was likewise obtained and a measurable melt index even at a pressure :as low as 1000 p.s.i.g. (Table Il below).

The etect of pretreating the mixed TiCl3 and aluminum triethyl catalyst with butene-l prior to polymerization is illustrated in Table V below which includes Run 11 from Table IV. The procedure was to add the mixed catalyst to the reactor followed by adding the butane TABLE Il Grains Time, H2 (mole Final Yield, Lbs/lb. Melt Ti Left in Run TiGls hrs. percent Al/Ti grains of Ti Index Polymer,

in feed) Ratio p.p.in.

9 diluent (liquid) which was thereafter heated to the desired pretreat temperature. The butene-l was added at 50 C. to the butane and mixed catalyst and the mixture heated to 65 C. for from 1 to 10 minutes.

As seen from above in Runs 11 and 13, which contain less than p.p.m. Ti on the final resin, higher catalyst eiciencies were more easily obtained than where no butene-l pretreatment occurred.

Rheological property measurements of several polymers prepared as above and comparison of these properties with those of commercial linear polyethylene resins indicated that preparation of the polyethylene by the relatively higher pressures as set forth herein did not change molecular weight distribution and that processability thereof is comparable to commercial resins. Thus, evaluation of several resins, including those from Runs 3 and 8, for flow properties in a high -shear viscometer demonstrated that these polymers have flow properties which are similar to commercial linear polyethylenes prepared at lower pressures. Essentially, therefore, none of the good properties of these resins is adversely affected by operation at the extremely high productivities illustrated.

Copending application, Serial Number 299,030 iiled even date herewith illustrates a process for the preparation of linear polyethylenes of low wax content and of densities above 0.955 at lower pressures than illustrated herein employing inert diluents such as propane and butane.

Modications can be made to the process herein falling within the scope of the appended claims.

What is claimed is:

1. A high catalyst eihcency process for preparing an ethylene polymer having a density of at least 0.955 which comprises reacting ethylene in a polymerization reactor at pressures of at least 800 p.s.i.g. and temperatures of from 50 to 100 C. in the presence of an inert hydrocarbon diluent boiling below 10 C., in liquid form and hydrogen in amounts of from 10 to 80% based on the ethylene feed employing as the catalyst for the polymerization .a crystalline cocrystallized titanium trichloride having the formula nTiCl3AlCl3 Where n is a digit of 1 to 5 `activated with an aluminum trialkyl, the aluminum to titanium ratio of said catalyst employed being at least 25:1, the ethylene polymer being produced with a catalyst eiliciency of at least about 60,000 pounds of polymer per pound of catalyst based on titanium metal.

2. The process of claim 1 wherein the aluminum trialkyl compound is aluminum triet-hyl and the ethylene polymer produced has a density of at least 0.96.

3. The process of claim 1 wherein butane is employed as the inert hydrocarbon diluent.

4. A process for polymerizing ethylene in a high catalyst eiciency process which consists essentially of the steps of (a) introducing ethylene to a polymerization reactor containing therein an inert hydrocarbon diluent boiling below 10 C. at pressures of at least 800 p.s.i.g. and temperatures of from about 50 to 100 C.,

(b) polymerizing said ethylene in the presence `of hydrogen in amounts of from 10 to 80% based on the ethylene feed and in the presence of a crystalline cocrystallized titanium trichloride having the formula nTiC13-A1Cl3 where n is a digit of 1 to 5 activated with an aluminum trialkyl compound, the aluminum to titanium ratio employed being =at least 25: 1- and up to :1,

(c) withdrawing from said reactor a slurry of ethylene polymer, unreacted ethylene and hydrocarbon diluent to a low pressure zone maintained at a pressure of from 0 to 50 p.s.i.g. and

(d) flashing from said slurry unreacted ethylene and hydrocarbon diluent in said low pressure zone, and

(e) recycling said unreacted ethylene and hydrocarbon diluent to said reaction zone and recovering ethylene polymer, said ethylene polymer being produced with `a catalyst efficiency of at least about 60,000 pounds of polymer .per pound of catalyst based on titanium metal.

5. The process of claim 4 wherein butane is employed as the inert hydrocarbon diluent.

6. A high catalyst etliciency process for preparing homopolymers of ethylene having a density of 0.96 and above which consists essentially in reacting ethylene in a polymerization reactor at pressures of 800 to 10,000 p.s.i.g. and temperatures `of from 50 to 100 C. in the presence of an inert hydrocarbon diluent in liquid form,

said hydrocarbon having a boiling point below 10 C., and hydrogen in amounts of from 10 to 80% based on the ethylene, said reaction being carried out in the presence of a crystalline titaniumv trichloride cocrystallized with `aluminum chloride and aluminum trialkyl compound, the aluminum to titanium ratio being at least 25:1, the ethylene homopolymer being produced with a catalyst eiciency of at least about 60,000 pounds of polymer per pound of catalyst based on titanium metal.

7. The process of claim 6 wherein butane is employed as t-he inert hydrocarbon diluent.

References Cited by the Examiner UNITED STATES PATENTS 2,893,984 7/ 1959 Seelbach 260*93.7 2,990,399 6/ 1961 Peterlein 260-93.7 12,996,459 8/1961` Andersen et al. 2610-949 3,032,509 5/ 1962 Langer et al. 260-94.9 3,046,266 7/ 1962 Benning et al. 260-94.9 3,051,690 8/1962I Vandenberg 260*94.9 3,126,365 3/ 1964 Hooker 260-94.9 3,225,021 12/ 1965 Erchak 260-949 FOREIGN PATENTS 538,782 12/1955 Belgium.

JOSEPH L. SCHOFER, Primary Examiner. F. L. DENSON, Assistant Examiner. 

1. A HIGH CATALYST EFFICIENCY PROCESS FOR PREPARING AN ETHYLENE POLYMER HAVING A DENSITY OF AT LEAST 0.955 WHICH COMPRISES REACTING ETHYLENE IN A POLYMERIZATION REACTOR AT PRESSURES OF AT LEAST 800 P.S.I.G. AND TEMPERATURES OF FROM 50* TO 100*C. IN THE PRESENCE OF AN INERT HYDROCARBON DILUENT BOILING BELOW 10*C., IN LIQUID FORM AND HYDROGEN IN AMOUNTS OF FROM 10 TO 80% BASED ON THE ETHYLENE FEED EMPLOYING AS THE CATALYST FOR THE POLYMER- 